Shape and displacement patterns of the gerbil tympanic membrane in experimental otitis media with effusion

This study assesses the visco-elastic properties of the tympanic membrane (TM) in isolated gerbilline temporal bones as a function of time after inducing experimental otitis media with effusion (OME). To do this we measured the TM displacements produced by application of sequences of static pressures across the TM, with a high resolution, real-time, differential moiré interferometer, and the results were compared with measurements on healthy ears. Two methods of producing OME were used: in one group tubal plugging was performed to produce mild OME (the 'TP group'); in the other group electro-cauterization of the nasopharyngeal orifice of the Eustachian tube was used to cause a severe form of OME (the 'EC group'). The measurements were performed from one day up to ten weeks after surgery. In the TP group the displacement fringe patterns were normal, i.e. qualitatively they resembled the patterns of the control group. Quantitatively there was a significant decrease of displacement for a given pressure on the first day after surgery, followed by a trend of increase with time; after seven to ten days the displacement was larger than in the control group. In the EC group the displacement was significantly reduced after half a week, followed by a trend of increase with time, similar to what was found in the TP group; at one week the displacement was larger than in the control group, and at ten weeks the largest displacement was recorded. In the EC group the displacement patterns were often irregular; in some cases with changes suggesting the presence of weak spots in the TM where retraction pockets most likely could develop. OME seems to affect the stiffness of the TM promptly so that it is a potential parameter for early diagnosis. The stiffness changes may, if measurable in the clinical situation, become prognostic parameters in the treatment of OME.